Lithium primary batteries use lithium and other light metals as a negative electrode active material, and manganese dioxide or graphite fluoride as a positive electrode active material. These batteries have unique features including high voltage, high energy density, low self-discharge, and an extremely long storage life, and hence are used in various electronic devices.
Among the materials used as a positive electrode active material, manganese dioxide is very popular because it is inexpensive and abundant, and as a positive electrode active material for lithium primary batteries, electrolytic manganese dioxide is commonly used due to its excellent discharge performance and long-term storage performance.
Electrolytic manganese dioxide is generally electrolytically synthesized in a sulphuric acid solution containing manganese ions, and therefore, is required to be neutralized with alkali when used as a positive electrode active material for lithium primary batteries. Popular examples of the alkali used for neutralization are ammonia and sodium hydroxide.
Electrolytic manganese dioxide prepared by neutralization with ammonia (hereinafter, ammonia-neutralized product) is widely used for lithium primary batteries. The ammonia-neutralized product, however, is manufactured by only a few manufacturers, and therefore, is less available and more expensive than electrolytic manganese dioxide prepared by neutralization with sodium hydroxide (hereinafter, sodium-neutralized product). As another disadvantage, when used as a positive electrode active material for lithium primary batteries, the ammonia-neutralized product requires a dedicated exhaust system to ensure working conditions because it causes ammonia to volatize, giving off a pungent smell when heat-treated to remove moisture.
The sodium-neutralized product, on the other hand, is mainly used as a positive electrode active material for dry batteries. The sodium-neutralized product generally contains 0.3 to 0.5% by mass of sodium, which may reduce the discharge performance when used for lithium primary batteries. The reason for the reduction is that the sodium in the sodium-neutralized product is deposited on the lithium used as a negative electrode active material and forms a resistance film thereon. The deposition is more significant as the battery is stored at a higher temperature and for a longer period. This is why the sodium-neutralized product is little used as a positive electrode active material for lithium primary batteries although it is easily available.
The sodium-neutralized product, however, is inexpensive and mass-produced, and therefore, it is a valuable attempt from an industrial viewpoint to make full use of this product as electrolytic manganese dioxide for lithium primary batteries.
To achieve this attempt, it has been suggested that electrolytic manganese dioxide is sodium-neutralized in such a manner that the neutralized electrolytic manganese dioxide has a minimum sodium content in the range of 0.05 to 0.2% by mass (Patent Literature 1, for example).
The sodium-neutralized product having a minimum sodium content, however, contains a large amount of sulfuric acid residues, making its pH as low as 2 to 4. When sintered and used as a positive electrode active material for lithium primary batteries, such a sodium-neutralized product with a low pH increases the battery internal resistance when a weak discharge is continued for a long time such as over one year, although its initial discharge performance is excellent.    Patent Literature 1: Japanese Patent Unexamined Publication No. 2001-236957